The present invention relates to a booster circuit for use in an electronic watch having an electroluminescent element (referred to as an EL element hereafter) and a piezo-electric alarm device using a piezo-electric element.
Conventionally, a circuit shown in FIG. 1(A) is used as a booster circuit for a piezo-electric alarm device. A boosting coil 2 is connected in series with a transistor 3 and connected in parallel with a piezo-electric element 1.
When a signal having a waveform `a` shown in FIG. 1(B) is fed to a base of the transistor 3, a voltage having a waveform `b` shown in FIG. 1(B) develops at a connection point `b` of the boosting coil 2 and the transistor 3. A peak voltage of the voltage waveform `b` of approximately 10 V is sufficient to generate an alarm sound when applied to the piezo-electric element.
FIG. 2(A) shows a conventional EL element driver. A boosting coil 6 is connected in series with a transistor 9, and one terminal of a diode 10 is connected with a connecting point of the boosting coil 6 and the transistor 9 and the other terminal of the diode 10 is connected with a connecting point `e` of an EL element 5 and a transistor 8. A resistor 7 is connected as shown for limiting currents.
The boosting principle will now be illustrated. When a waveform `c` in FIG. 2(B) is fed to a base of the transistor 9, a pulse counter electromotive voltage develops across the boosting coil 6 at the fall timing of each pulse. The counter electromotive voltage is charged in the EL element 5 having the equivalent electric capacity via the diode 10. The terminal `e` rapidly boosts by repeating the above operation several times (three times in the figure), and the voltage of approximately 100 V is obtained. The transistor 8 is then turned on by applying a signal `d` to a base of the same, and the EL element is luminesced by discharging the accumulated electric charges.
The conventional EL element driver is disadvantageous in that it is necessary to mount two boosting coils of large volume for boosting both an EL element and a piezo-electric element on an electronic watch. As a result the overall watch size is increased and the cost of the watch is increased.